The art of lithographic printing is based upon the immiscibility of oil and water, wherein the image area preferentially retains the oily material or ink. When a suitably prepared surface is moistened with water and ink is then applied, the background or non-image area retains the water and repels the ink while the image area accepts the ink and repels the water. The ink on the image area is then transferred to the surface of a material upon which the image is to be reproduced; such as paper, cloth and the like. Commonly the ink is transferred to an intermediate material called the blanket, which in turn transfers the ink to the surface of the material upon which the image is to be reproduced.
A very widely used type of lithographic printing plate has a light-sensitive coating applied to an aluminum base support. The coating may respond to light by having the portion that is exposed become soluble so that it is removed in the developing process. Such a plate is referred to as positive-working. Conversely, when the portion of the coating that is exposed becomes hardened, the plate is referred to as negative-working. In both instances the image area remaining is ink-receptive or oleophilic and the non-image area or background is water-receptive or hydrophilic. The differentiation between image and non-image areas is made in the exposure process where a film is applied to the plate with a vacuum to insure good contact. The plate is then exposed to a light source, a portion of which is composed of UV radiation. In the instance where a positive plate is used, the area on the film that corresponds to the image on the plate is opaque so that no light will strike the plate, whereas the area on the film that corresponds to the non-image area is clear and permits the transmission of light to the coating which then becomes more soluble and is removed. In the case of a negative plate the converse is true. The area on the film corresponding to the image area is clear while the non-image area is opaque. The coating under the clear area of film is hardened by the action of light while the area not struck by light is removed. The light-hardened surface of a negative plate is therefore oleophilic and will accept ink while the non-image area which has had the coating removed through the action of a developer is desensitized and is therefore hydrophilic.
In the field of preparing lithographic and photopolymer letterpress printing plates, light sensitive coatings are placed upon a support, usually a flexible, thin metal sheet, thus creating a presensitized printing plate. The plate is subjected to imagewise exposure through a mask which renders the photosensitive substance insoluble in a suitably chosen developer, if it is negative working; and soluble in the developer, if the plate is positive working. The action of the developer, in the case of a lithographic plate, is to remove the non-image portions of the plate coating completely so that no trace of it remains on the metal support. The metal support, so revealed, is now free of coating and will no longer accept ink. This sharp discrimination between image and non-image areas on the plate is vital to success in the preparation of a lithographic plate. Exposed lithographic plates may be developed by hand or by machine. If developed by hand, developer is poured both upon the plate and upon a soft sponge or pad and rubbed therewith upon the surface of the plate until, by inspection, the non-image areas are completely removed. Care must be taken to avoid any damage to the image by excessive rubbing, or by the use of an overly active chemical developer. When many plates are to be developed, machinery is used which will develop the plate by applying developer mechanically. In some existing machines, developer is metered to the plate through tubes and spread about by sponges or brushes which also serve to separate loosened non-image particles from the plate support. In a next stage, within the machine, the spent developer and separated non-image coating are rinsed from the plate, usually with water, supplied through tubes. In a final stage, the rinsed, moist surface is covered with a gum solution and any excess thereof is removed, thereby delivering a plate ready for the press or for storage. The gum solution is also metered to the plate through tubes. In some cases, these processing solutions are pumped through spray nozzles that are directed upon the plate surface.
Several different types of plate processors are available, differing primarily in the manner in which the plate is washed (rather than the manner of drying or post-curing). In one approach, the plate is moved through a washing region while the sprays from a large number of low-pressure spray nozzles are directed against the plate. The quality of the finished plates is good, but a large amount of surface foam is generated that is difficult to suppress. This machine can have a high throughput of plates, but is expensive to operate. Another plate processor uses reciprocating brushes to scrub away the unreacted photopolymer while the plates are passed through a shallow water bath. Most of the unreacted photopolymer material is removed, but the bristles may not be able to reach into small recesses, which may be smaller than the diameter of a single bristle. Unreacted photopolymer material may consequently be left on the surface of the printing plate, particularly in the small recesses of raised areas. The result is a buildup of ink between these raised areas during printing, and a progressive deterioration in print quality. The scrubbing action also can have the unfortunate side effect of damaging some of the polymerized areas, which desirably should not be affected by the washing, by rubbing away some of the polymerized material. The existing plate processors therefore produce plates that may have unremoved photopolymer or damage to the relief areas, both of which reduce print quality. In addition, they are large in size, using valuable floor space, and are costly to purchase and to operate.
U.S. Pat. No. 3,955,433 describes an automatic self-contained basic unit-processor for an offset photolithographic plate which may be used, at different times, for any processing step requiring contacting of the plate by liquid. For example, an exposed plate is developed by contacting the exposed surface of the plate with the conventional developing fluid, and subjecting the surface to a linear, reciprocating rubbing action by a sponge. This rubbing action is claimed to produce results surprisingly similar to those produced by a manual rotary rubbing action.
U.S. Pat. No. 3,943,539 describes a vertical plate processor, in which dwell time, brushing and squeegee for developing plates which are inserted vertically from the top of the processor between pairs of transport rollers and brush rollers and thereby immersed in a developing solution are provided. After a pre-set dwelling time, a drive motor is activated for rotating the rollers to deliver the plate up out of the processor. The processor provides automatic developing of the plate in a vertical, non-feedthrough operation, thereby permitting a compact apparatus requiring a minimum of table top space. Again, development is done by brushing with rollers.
U.S. Pat. No. 3,936,853 shows a processor in which the developer members may comprise either sponges or brushes mounted for engagement with the exposed surface of the lithographic plate, again requiring rubbing contact for the development process.
U.S. Pat. No. 3,937,175 describes machinery for the processing of exposed lithographic and photopolymer printing plates, wherein the plate is transported under spray heads through which developing solutions, rinses, and finishing solutions, as determined by the needs of the individual plate, are sequentially dispensed. Appropriate time intervals are provided for the action of each solution, optionally assisted by soft non-scratching brushes, scrubbing pads, squeegees and the like. The various manifolds to which the spray nozzles are attached are equipped with pumps and solenoid valves electrically actuated by a timer mechanism. The timer is adjusted to cause the valve to open and close rapidly. During the closed phase of the cycle, the fluid pressure builds up to the ultimate capability of the pump. Upon opening of the valve, the fluid, under pressure, is suddenly ejected upon the target to accomplish the impingement on and penetration into the target beyond that which would have been obtained with continuous spraying. The effects of this improved impingement and penetration are to clean out unwanted image areas more quickly and thoroughly while at the same time reducing the volume of needed developer.
U.S. Pat. No. 4,222,656 shows an apparatus for processing exposed lithographic plates in which an exposed plate is continuously fed forward with the exposed face of the plate facing up, developer is delivered onto the upper face of the plate by a nozzle which is traversed back and forth laterally with respect to the path of the plate above the plate to distribute the developer across the upper face of the plate, the developer is rubbed over the upper face of the plate, the plate is sprayed with water, preservative is delivered onto the upper face of the plate by a nozzle which is traversed back and forth along with the developer delivery nozzle to distribute the preservative across the upper face of the plate, and the preservative is rubbed over the upper face of the plate. The plate is fed forward by sets of plate feed rolls.
U.S. Pat. No. 5,349,413 describes a sheet-like material processing apparatus, in which there is no rubbing, but the solution is contained by squeeze rollers which contact the surface.
U.S. Pat. No. 5,223,041 Apparatus and process for processing printing plates using high pressure water spray water to the water spray nozzle under a pressure of from about 500 to about 1000 pounds per square inch.
Several different types of plate processors are available, differing primarily in the manner in which the plate is washed (rather than the manner of drying or post-curing). In one approach, the plate is moved through a washing region while the sprays from a large number of low-pressure spray nozzles are directed against the plate. The quality of the finished plates is good, but a large amount of surface foam is generated that is difficult to suppress. This machine can have a high throughput of plates, but is expensive to operate. Another plate processor uses reciprocating brushes to scrub away the unreacted photopolymer while the plates are passed through a shallow water bath. Most of the unreacted photopolymer material is removed, but the bristles may not be able to reach into small recesses, which may be smaller than the diameter of a single bristle. Unreacted photopolymer material may consequently be left on the surface of the printing plate, particularly in the small recesses of raised areas. The result is a buildup of ink between these raised areas during printing, and a progressive deterioration in print quality. The scrubbing action also can have the unfortunate side effect of damaging some of the polymerized areas, which desirably should not be affected by the washing, by rubbing away some of the polymerized material. The existing plate processors therefore produce plates that may have unremoved photopolymer or damage to the relief areas, both of which reduce print quality. In addition, they are large in size, using valuable floor space, and are costly to purchase and to operate.
U.S. Pat. No. 5,136,322 describes a light-sensitive material processing apparatus for processing an image-wise exposed light-sensitive material by immersing the light-sensitive material in a processing solution while the light-sensitive material is being automatically conveyed. The apparatus includes a scraping brush constituted by a shaft and a synthetic resin-made elongated brush wound spirally around an outer periphery of the shaft. Accordingly, the light-sensitive layer of the light-sensitive material is scraped off by the scraping brush to effect processing.
U.S. Pat. No. 4,555,302 shows a method and apparatus for etching light-sensitive photopolymerizable surfaces of printing plates. The apparatus includes a bath of etching fluid (32), a conveyor (38, 40, 42, 52) for conveying printing plates (10) in a prescribed path (54), and transducers (46) mounted above the plates as they are conveyed within the bath and capable of forming longitudinal waves in the etching fluid at generally right angles to the surface of the printing plates. The invention is characterized by directing processing fluid between the acoustical radiating surface of the transducers (46) and the surface of the plates (10) in such a manner that the surface to be etched is swept with processing fluid of a sufficient velocity and volume as to prevent substantial cavitational erosion of the surface to be etched.
U.S. Pat. No. 5,663,037 provides a direct write lithographic printing plate which is imaged by a high intensity laser beam, causing the exposed areas to become more soluble than the background when processed with a rotating brush in a processing solution.
There remains a need for a direct write lithographic printing plate that can be processing without rubbing contact.